1. Field of the Invention
The present invention relates to a display system, and more particularly, to a display system disposing a charge pump circuit on a flexible printed circuit (FPC) externally coupled to its display device for improving its voltage converting efficiency.
2. Description of the Prior Art
A charge pump is a type of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pumps use some form of switching devices to control the connection of voltages to the capacitor. The charge pumps can also double voltages, triple voltages, halve voltages, invert voltages, fractionally multiply or scale voltages such as × 3/2, × 4/3, ×⅔, etc. and generate arbitrary voltages, depending on the controller and circuit topology. A traditional charge pump circuit includes a voltage source, one or more charge capacitances, a load capacitance, a number of circuit switches and a fixed-frequency clock used to control the circuit switches. Using a clock period as an example (e.g. a doubled two phase circuit), in the first half period, circuit switches are used to make a parallel connection between a voltage source and a charge capacitance so as to charge the charge capacitance to a voltage level; in the second half period, circuit switches are used to make a serial connection between the voltage source and the charge capacitance and a load capacitance. After a number of periods are repeated, the voltage difference between two sides of the load capacitance will be lifted up to a voltage level that is much higher than that of the original voltage source.
In traditional small-sized and medium-sized thin-film transistor liquid crystal display (TFT-LCD) devices, with the growing size of the screen, the current consumption is also growing. If the charge pump circuit is disposed in the driving circuit of the TFT-LCD device, its voltage converting efficiency will get worse due to being limited by the indium tin oxide (ITO) resistors.
In addition, since the system end hopes to provide an input voltage ranging from 2.0V to 4.8V to the driving circuit of the TFT-LCD device directly, the charge pump circuit should be able to support a voltage converting ratio with different multiples (such as 1.5 times, 2 times, or 3 times) to provide the desired output voltage. Therefore, an important research and development subject in the industry is how to dispose a charge pump circuit in the TFT-LCD device without it being affected by the ITO resistors, and how to control the charge pump circuit.